Flow-through capacitors have been described in the prior art (see for example U.S. Pat. Nos. 5,360,540, 5,192,432, 5,196,115, 5,200,068 and 5,415,768 to Andelman; U.S. Pat. No. 3,658,674 to Benak; and PCT International Application No. US94/05364 to Andelman. The publication by Allen M. Johnson et al "The Electrosorb Process for Desalting Water", Mar. 1970, The Office of Saline Water Research and Development Progress Report No. 516, U.S. Department of Interior PB 200 056 and U.S. Pat. No. 5,425,858 to Joseph Farmer describe flow-through capacitors or flow-through deionization systems.
Flow-through capacitors of the prior art function by concentration of solutes in the feed stream into a concentrated waste stream. This has two serious disadvantages. The first disadvantage is that the method of waste recovery into a concentrated waste stream generates waste water. Waste water of any kind is often a significant process cost.
Another disadvantage of the prior art is tendency to foul. Solutes present in many ambient waters, such as Calcium Sulfate or Calcium Carbonate or other solutes, are present at or near saturation. When concentrated past the saturation point, such solutes have a tendency to form scales and foul the apparatus. Fouling is a well known problem in waste water apparatus, such as reverse osmosis and electrodialysis, that share with the flow-through capacitor the tendency to concentrate solutes past the saturation point.
Commonly present turbidity, colloids, and particles may also block flow channels and form deposits. Benak, U.S. Pat. No. 3,658,674, describes a flow-through capacitor that rapidly plugs up when used with hard well water (column lines 66-68). He further describes (page 5, lines 27 through 37) problems with the purification of Calcium Sulfate. Prior art U.S. Pat. Nos. 5,360,540, 5,192,432, 5,196,115, 5,200,068 and 5,415,768 by Andelman, U.S. Pat. No. 3,658,674, the publication by Allen M Johnson et al "The Electrosorb Process for Desalting Water", Mar. 1970, The Office of Saline Water Research and Development Progress Report No. 516, U.S. Department of Interior PB 200 056, and U.S. Pat. No. 3,658,674 by Benak all describe flow-through capacitors with a porous spacer layer. The pores in the spacer layer can be clogged up and fouled by turbidity already present in the feed stream, as well as by precipitates and crystals that form during use.
U.S. Pat. No. 5,425,858 by Joseph Farmer describes a flow-through capacitor whose spacer layers define an open channel with a long serpentine flow path. In the Farmer patent, the flow path is not also directly open to the outside, but is circumscribed by a gasket and forced to flow between holes in the successive multiple structural layers, and thence through an outlet. Serpentine channels provide bends where crystal and precipitates can settle, thereby blocking the flow path. Fluid flow-through holes in many successive structural layers further constricts the flow and offers many places for fouling to occur and multiple opportunity for solids to plug the flow path. A short flow path is preferable in order to flush the saturated waste out of the capacitor before the kinetic process of crystallization forms precipitates and causes fouling or crystals. The long flow path that a serpentine channel provides makes it difficult to flush the capacitor before crystallization occurs. Once solids do form inside the capacitor, it is that much more difficult to flush them through a long enclosed flow path.
The Farmer patent suffers from other disadvantages. The plate frame design depends upon multiple sealing gaskets. This provides multiple opportunities for leaks, and therefore requires hardware such as heavy, structural, metal end plates and threaded rods to tightly compress the stack. The structural, metallic end plates are conductive, and therefore, in addition to the gasket spacers, require an extra non-conductive insulator layer between the end electrodes and the end plate. The electrodes in the Farmer device consist of a titanium metal sheet sandwiched between two high surface area layers. To effect leak tight seals, the titanium metal sheets also have to be thick and structural, rather than the thin metal foils described in the Andelman patents. This adds to the cost and bulk of the system. Moreover, titanium is not the best choice, as titanium is a valve metal, and forms a non-conductive oxide coating under conditions which may occur during use of the flow-through capacitor. Finally, the Farmer device uses a conductive epoxy rather than a compression contact between the titanium sheets and the high surface area materials. This unnecessarily increases resistance of the electrical contact. The use of glue to form a contact would also shorten the lifetime of the capacitor device due to eventual deterioration of the bond.
There is therefore a significant and unfulfilled need for a new and improved flow-through capacitor apparatus, method and system. Such a capacitor would be resistant to fouling. It would furthermore be desirable that such a capacitor reduce or even eliminate the waste water entirely. Finally, it would be desirable for such a capacitor to be easy to manufacture, and not have any unnecessary parts that increase cost, or limit the usefulness of the capacitor.